1. Field of the Invention
The present invention relates to controlling access to a shared storage system.
2. Description of the Related Art
Concerns that commonly arise in clustered environments that relate to controlling access to shared resources are:
1. Establishing which cluster members currently have access to shared resources; and
2. When one cluster member is lost, guaranteeing to the surviving members of the cluster that both:                (i) The lost member is prevented from having future access to the shared resource without said lost member becoming aware that it needs to interlock with the rest of the cluster; and        (ii) Ensuring that any use of the shared resource by the lost member is committed or aborted, so that the surviving members can perform recovery without being undermined by stale traffic from the lost member.        
Part 2 of the problem (including both sub-parts (i) and (ii) is sometimes known as “fencing”.
Managing the cluster membership has a number of solutions. One means might be to record the membership in some location which has adequate availability.
The fencing problem is also addressed by a number of techniques. One solution where the shared resource is a SCSI-3 block device is to use Persistent Reserve (PR). Specifically this provides a ‘PREEMPT & ABORT’ sub-command that both removes the lost cluster member, thus denying said lost cluster member future access to the said SCSI-3 block device, and further commits or aborts its ongoing I/O so that the shared resource is stable for use by the surviving cluster members.
In a system such as V7000 Unified, there are many such logical units. There are in principle two ways of structuring the solution using SCSI PR:                Maintain a cluster membership list on all LUs, and use PR including PREEMPT&ABORT on all volumes.        Use PR and PREEMPT&ABORT on just one volume.        